Intense Pump Pulse Research Articles

Observation of the optical –A2Πu coupling in lasing induced by intense laser field**Project supported by the National Natural Science Foundation of China (Grant Nos. 61625501, 11904121, and 61427816), the Open Fund of the State Key Laboratory of High Field Laser Physics (SIOM), China, the Program for JLU Science and Technology Innovative Research Team (JLUSTIRT), China (Grant No. 2017TD-21), and Fundamental

We propose a simple pump-coupling-seed scheme to examine the optical –A2Πu coupling in lasing. We produce the lasing at 391 nm, corresponding to the (v′ = 0)–(v = 0) transition, by externally seeding the gain medium prepared by irradiation of N2 with an intense pump pulse. We then adopt a weak coupling pulse in between the pump and seed pulses, and show that the intensity of the 391-nm lasing can be efficiently modulated by varying the polarization direction of the coupling pulse with respect to that of the pump pulse. It is found that when the polarization directions of the pump and coupling pulses are perpendicular, the 391-nm lasing intensity is more sensitive to the coupling laser energy, which reflects the inherent nature of the perpendicular –A2Πu transition.

Unveiling the mechanism of wave-front distortion of superfluorescent pulses

We investigated the transverse properties of 420-nm yoked superfluorescence (YSF) emitted from the atomic vapor of rubidium by driving the $5S\text{\ensuremath{-}}5D$ two-photon transition with an ultrashort laser pulse. From the observed interferograms of the superfluorescent pulses, we derived both the amplitude and phase properties of their electric fields. By increasing the pump-pulse intensity, the spatial profile of the 420-nm YSF changed from a central bright spot into a ring-shaped radial profile, accompanied by a simultaneous distortion of the wave front. This behavior can be explained by considering the intensity-dependent phase shifts of the $5S\text{\ensuremath{-}}5D$ wave packets during the excitation process, which was further imprinted to the 420-nm YSF wave front. The single-shot experiments enabled us to clarify the locally coherent regions of the 420-nm YSF as well as their stochastic feature.

Ionization and Neutralization Dynamics of CsPbBr3 Perovskite Nanocrystals Revealed by Double-Pump Transient Absorption Spectroscopy.

Ionization of nanocrystals (NCs) causes both photoluminescence intermittency and a reduction in luminescence quantum efficiency and thus plays a critical role in the optoelectronic performance of NC-based devices. Here, we study the ionization and neutralization processes of CsPbBr3 perovskite NCs under strong photoexcitation by means of double-pump transient absorption spectroscopy. A strong initial pulse is used to generate ionized NCs, and their optical responses are investigated by varying the excitation intensity and delay time of the second pump pulse. We find that charging can occur either via nonradiative Auger recombination of biexcitons or via any possible recombination of trions. The presence of the extra charge inside of an ionized perovskite NC significantly reduces its absorption cross section. The experiments reveal that ionized NCs exhibit two types of neutralization processes with time constants on the order of nanoseconds and microseconds. These results are useful for the optimal design of NC-based photonic devices.

All optical control of comb-like coherent acoustic phonons in multiple quantum well structures through double-pump-pulse pump-probe experiments.

We present an advancement in applications of ultrafast optics in picosecond laser ultrasonics - laser-induced comb-like coherent acoustic phonons are optically controlled in a In0.27Ga0.73As/GaAs multiple quantum well (MQW) structure by a high-speed asynchronous optical sampling (ASOPS) system based on two GHz Yb:KYW lasers. Two successive pulses from the same pump laser are used to excite the MQW structure. The second pump light pulse has a tunable time delay with respect to the first one and can be also tuned in intensity, which enables the amplitude and phase modulation of acoustic phonons. This yields rich temporal acoustic patterns with suppressed or enhanced amplitudes, various wave-packet shapes, varied wave-packet widths, reduced wave-packet periods and varied phase shifts of single-period oscillations within a wave-packet. In the frequency domain, the amplitude and phase shift of the individual comb component present a second-pump-delay-dependent cosine-wave-like and sawtooth-wave-like variation, respectively, with a modulation frequency equal to the comb component frequency itself. The variations of the individual component amplitude and phase shift by tuning the second pump intensity exhibit an amplitude valley and an abrupt phase jump at the ratio around 1:1 of the two pump pulse intensities for certain time delays. A simplified model, where both generation and detection functions are assumed as a cosine stress wave enveloped by Gaussian or rectangular shapes in an infinite periodic MQW structure, is developed in order to interpret acoustic manipulation in the MQW sample. The modelling agrees well with the experiment in a wide range of time delays and intensity ratios. Moreover, by applying a heuristic-analytical approach and nonlinear corrections, the improved calculations reach an excellent agreement with experimental results and thus enable to predict and synthesize coherent acoustic wave patterns in MQW structures.

Fourier transform vibrational spectroscopy of D2+ by few-cycle near-infrared laser pulses

By strong-field Fourier transform spectroscopy using intense pump and probe few-cycle laser pulses, the vibrational level separations of D2+ were determined with high precision by taking advantage of the dressed-state formation by the probe pulse.

Insights into the Femtosecond to Nanosecond Charge Carrier Kinetics in Perovskite Materials for Solar Cells

In this work, we analyze a few simple methods that permit getting insight into the charge dynamics in methylammonium lead iodide (MAPbI3) perovskites, on the basis of femtosecond transient absorption and fluorescence studies. We show that proper determination of the charge population decay kinetics requires broad spectral integration of the long-wavelength bleach signal (∼760 nm) assigned to the band-filling effect, which can be realized, e.g., by band integral analysis. On the contrary, the kinetics related to a shorter-wavelength bleach band (∼480 nm) behave differently: a single-wavelength decay is sufficient to probe charge population. Moreover, both the shorter-wavelength excitation and higher pump pulse intensities result in significantly longer charge cooling times, which might be important in efficient hot carrier extraction. The validity of our previously proposed easy approach for the elimination of higher-order recombination rates is also discussed and confirmed in terms of proper estimation of...

Energy Scaling, Second Stokes Oscillation, and Raman Gain-Guiding in Monolithic Diamond Raman Lasers

Energy scaling of the first Stokes oscillation is compared in micro-lensed and plane-plane monolithic diamond Raman lasers under 532-nm pumping. A maximum Raman pulse energy of $92~\mu \text{J}$ at 573 nm was achieved with the micro-lensed device, whereas in a plane-plane configuration, the maximum Raman pulse energy was 3.1 mJ. The second Stokes generation at 620 nm in 2 and 1 mm long micro-lensed monolithic diamond Raman lasers is also reported. The best conversion efficiency from the pump at 532 nm, namely, 63%, was observed in a 2 mm long crystal at the pump pulse intensity of 4.5 GW/cm2. By measuring the output Raman laser beam caustic, it was found that the second Stokes intracavity beam radius at the output coupler of the micro-lensed device is at least two times smaller than that expected from the ABCD matrix calculations of the resonator mode. A Raman gain-guiding mechanism is suggested to explain this difference.

Scaling for ultrashort pulse amplification in plasma via backward Raman amplification scheme operating in the short wavelength regime

The applicability of extreme ultraviolet (XUV) and soft x-ray radiation sources in a resonant backward Raman amplification (BRA) scheme in amplifying the weak ultrashort sub-femtosecond pulses has been examined. Utilizing the slowly-varying-envelope-approximation-based analytical expressions for the resonant three-wave interaction along with viable physics constraint, a parameter space configuring the laser/plasma features required to achieve significant pulse amplification and compression has been derived. A specific parametric configuration for resonant BRA operation in the short wavelength regime has been identified and validated using particle in cell simulation results. The outcome predicts that the resonant BRA scheme may efficiently be applied to amplify the weak pulses for the pump radiation operating within the XUV regime; the effect may be optimized via efficient tuning of the plasma density, pump pulse intensity, and wavelength of the interacting waves. As an illustration, resonant BRA scheme in reference to a DESY FLASH XUV laser source has been conceptualized, and a competent set of laser/plasma parameters in achieving significant amplification is configured. The limitations and plausible rectification for a practical execution of the resonant BRA have also been discussed.

Compact gain-saturated x-ray lasers down to 685 nm and amplification down to 585 nm

Plasma-based x-ray lasers allow single-shot nano-scale imaging and other experiments requiring a large number of photons per pulse to be conducted in compact facilities. However, compact repetitively fired gain-saturated x-ray lasers have been limited to wavelengths above λ=8.85 nm. Here we extend their range to λ=6.85 nm by transient traveling wave excitation of Ni-like Gd ions in a plasma created with an optimized pre-pulse followed by rapid heating with an intense sub-picosecond pump pulse. Isoelectronic scaling also produced strong lasing at 6.67 nm and 6.11 nm in Ni-like Tb and amplification at 6.41 nm and 5.85 nm in Ni-like Dy. This scaling to shorter wavelengths was obtained by progressively increasing the pump pulse grazing incidence angle to access increased plasma densities. We experimentally demonstrate that the optimum grazing incidence angle increases linearly with atomic number from 17 deg for Z=42 (Mo) to 43 deg for Z=66 (Dy). The results will enable applications of sub-7 nm lasers at compact facilities.

Cross-phase-modulation-induced temporal reflection and waveguiding of optical pulses

Cross-phase modulation (XPM) is commonly viewed as a nonlinear process that chirps a probe pulse and modifies its spectrum when an intense pump pulse overlaps with it. Here we present an alternative view of XPM in which the pump pulse creates a moving refractive-index boundary that splits the probe pulse into two parts with distinct optical spectra through temporal reflection and refraction inside a dispersive nonlinear medium. The probe even undergoes a temporal version of total internal reflection for sufficiently intense pump pulses, a phenomenon that can be exploited for making temporal waveguides. We investigate the practical conditions under which XPM can be exploited for temporal reflection and waveguiding. The width and shape of the pump pulses as well as the nature of the medium dispersion at the pump and probe wavelengths (normal versus anomalous) play important roles. The super-Gaussian shape of a pump pulse is particularly helpful because of the relatively sharp edges of the super-Gaussian shape. When the pump wavelength lies in the anomalous-dispersion regime, the pump pulse can form a soliton, whose unique properties can be exploited to our advantage. We also discuss a potential application of XPM-induced temporal waveguides for compensating for timing jitter.

Transient-absorption phases with strong probe and pump pulses

The quantum dynamics of a system of Rb atoms, modeled by a V-type three-level system interacting with intense probe and pump pulses, are studied. The time-delay-dependent transient-absorption spectrum of an intense probe pulse is thus predicted, simulating pump-probe experiments in which this is preceded or followed by a strong pump pulse. Numerical results are interpreted in terms of an analytical model based on interaction operators, which quantify the transformation undergone by the system under the action of an intense pulse. The oscillating features of the resulting transient-absorption spectra, due to the coupling of several excited states, are thus interpreted in terms of the atomic population and phase changes imposed by the pump and probe pulses. Strong-field-induced phases and their influence on the resulting transient-absorption spectra are thereby investigated for different values of pump and probe intensities and frequencies, focusing on the atomic properties which are encoded in the absorption line shapes for positive and negative time delays.

The role of the global phase in the spatio-temporal evolution of strong-coupling Brillouin scattering

The role of the global phase in the spatio-temporal evolution of the 3-wave coupled equations for backscattering is analyzed in the strong-coupling regime of Brillouin scattering. This is of particular interest for controlled backscattering in the case of plasma-based amplification to produce short and intense laser pulses. It is shown that the analysis of the envelope equations of the three waves involved, pump, seed, and ion wave, in terms of phase and amplitude fully describes the coupling dynamics. In particular, it helps understanding the role of the chirp of the laser beams and of the plasma density profile. The results can be used to optimize or quench the coupling mechanism. It is found that the directionality of the energy transfer is imposed by the phase relation at the leading edge of the pulse. This actually ensures continued energy transfer even if the intensity of the seed pulse is already higher than the pump pulse intensity.

Twisting Anderson pseudospins with light: Quench dynamics in terahertz-pumped BCS superconductors

We study the preparation (pump) and the detection (probe) of far-from-equilibrium BCS superconductor dynamics in THz pump-probe experiments. In a recent experiment [R. Matsunaga, Y. I. Hamada, K. Makise, Y. Uzawa, H. Terai, Z. Wang, and R. Shimano, Phys. Rev. Lett. {\bf 111}, 057002 (2013)], an intense monocycle THz pulse with center frequency $\omega \simeq \Delta$ was injected into a superconductor with BCS gap $\Delta$; the subsequent post-pump evolution was detected via the optical conductivity. It was argued that nonlinear coupling of the pump to the Anderson pseudospins of the superconductor induces coherent dynamics of the Higgs (amplitude) mode $\Delta(t)$. We validate this picture in a two-dimensional BCS model with a combination of exact numerics and the Lax reduction method, and we compute the nonequilibrium phase diagram as a function of the pump intensity. The main effect of the pump is to scramble the orientations of Anderson pseudospins along the Fermi surface by twisting them in the $xy$-plane. We show that more intense pump pulses can induce a far-from-equilibrium phase of gapless superconductivity ("phase I"), originally predicted in the context of interaction quenches in ultracold atoms. We show that the THz pump method can reach phase I at much lower energy densities than an interaction quench, and we demonstrate that Lax reduction (tied to the integrability of the BCS Hamiltonian) provides a general quantitative tool for computing coherent BCS dynamics. We also calculate the Mattis-Bardeen optical conductivity for the nonequilibrium states discussed here.

Resonant femtosecond stimulated Raman spectroscopy with an intense actinic pump pulse: Application to conical intersections.

We theoretically investigate the feasibility of characterizing conical intersections with time-resolved resonant femtosecond stimulated Raman spectroscopy (FSRS) using an intense actinic pump pulse. We perform nonperturbative numerical simulations of FSRS signals for a three-electronic-state two-vibrational-mode model, which is inspired by the S2(ππ*)-S1(nπ*) conical intersection in pyrazine. Our results show that moderately strong actinic pulses increase the intensity of vibrational fingerprint lines in FSRS transients. They facilitate the extraction of useful spectroscopic information by enhancing peaks revealing the coupling and tuning modes of the conical intersection.

Element Selective Probe of the Ultra-Fast Magnetic Response to an Element Selective Excitation in Fe-Ni Compounds Using a Two-Color FEL Source

The potential of the two-color mode implemented at the FERMI free-electron laser (FEL) source for pumping and probing selectively different atomic species has been demonstrated by time-resolved scattering experiments with permalloy (FeNi alloy) and NiFe 2 O 4 samples. We monitored the ultra-fast demagnetization of Ni induced by the pump FEL pulse, by tuning the linearly-polarized FEL probe pulse to the Ni-3p resonance and measuring the scattered intensity in the transverse magneto-optical Kerr effect geometry. The measurements were performed by varying the intensity of the FEL pump pulse, tuning its wavelength to and off of the Fe-3p resonance, and by spanning the FEL probe pulse delays across the 300–900 fs range. The obtained results have Photonics 2017, 4, 6 2 of 10 evidenced that for the case of NiFe 2 O 4 , there is a sensible difference in the magnetic response at the Ni site when the pump pulse causes electronic excitations at the Fe site.

Dissociation Dynamics and Electronic Structures of Highly Excited Ferrocenium Ions Studied by Femtosecond XUV Absorption Spectroscopy.

The dissociation dynamics of ferrocene are explored following strong field ionization using femtosecond time-resolved extreme ultraviolet (XUV) transient absorption spectroscopy. Employing transitions in the vicinity of the iron 3p (M2,3) edge, the dissociation is monitored from the point of view of the iron atom. With low strong field pump intensities (≈2 × 1013 W cm-2), only ferrocenium cations are produced, and their iron 3p absorption spectrum is reported. It very closely resembles the 3p spectrum of atomic Fe+ ions but is red-shifted by 0.8 eV. With the aid of time-dependent density functional theory calculations, the spectrum is assigned to a combination of doublet and quartet spin states of ferrocenium ions. Ionization with more intense strong field pump pulses (≥6 × 1013 W cm-2) leads predominantly to the prompt production of ferrocenium ions that dissociate to give the spectral signature of bare Fe+ ions within 240 ± 80 fs. Within the temporal resolution of the experiment (≈40 fs), no spectral intermediates are observed, suggesting that the dissociation process occurs directly from the excited ferrocenium ion and that the bonds between the iron center and both cyclopentadienyl rings are broken almost simultaneously in an asynchronous concerted decay process. No evidence of slower dissociation channels is observed at a pump-probe delay of 250 ps, suggesting that all energy is very rapidly routed into dissociative states.

Time-domain pumping a quantum-critical charge density wave ordered material

We determine the exact time-resolved photoemission spectroscopy for a nesting driven charge-density-wave (described by the spinless Falicov-Kimball model within dynamical mean-field theory). The pump-probe experiment involves two light pulses: the first is an ultrashort intense pump pulse that excites the system into nonequilibrium, and the second is a lower amplitude higher frequency probe pulse that photoexcites electrons. We examine three different cases: the strongly correlated metal, the quantum-critical charge density wave and the critical Mott insulator. Our results show that the quantum critical charge density wave has an ultra efficient relaxation channel that allows electrons to be de-excited during the pump pulse, resulting in little net excitation. In contrast, the metal and the Mott insulator show excitations that are closer to what one expects from these systems. In addition, the pump field produces spectral band narrowing, peak sharpening, and a spectral gap reduction, all of which rapidly return to their field free values after the pump is over.

Moderately strong pump-induced ultrafast dynamics in solution

The transient transmittance spectra of laser dye IR144 in methanol were investigated experimentally in the moderately strong pump-probe field. Observed emission spectra in the red edge of the incident-field bandwidth, created by resonant impulsive stimulated Raman scattering (RISRS), display significant nonlinear intensity dependence as the pulse intensity increases. Dynamic perspectives of RISRS spectra can be understood well in a wavepacket picture. The excitation of high vibrational levels in the ground electronic state leading to the redshift of emissions presents high dependence of the pump-pulse intensity and ultrafast dynamical features, mapping the spatial overlap and separation of ground and excited wave functions and resolving the ultrafast vibrational relaxation in the femtosecond regime.

Coherently controlled emissions |4P3/2,1/2〉 ↔ |4S1/2〉 from a femtosecond Λ-type excitation scheme in potassium atom

The combined excitation of high density potassium (K) vapour by 100 fs pump-coupling pulses is experimentally studied. The intense pump pulse excites the two-photon transition and internally generated emissions are initiated along the atomic paths: (path-1) and, (path-2). The temporally delayed coupling pulse coherently drives the transitions, in a Λ-type excitation scheme. The competing axial and conical emission components of the well-resolved transitions (D2 and D1 lines of K) are substantially enhanced and controlled, for appropriate detunings and pump-coupling temporal delays. The coherence relaxation time (CRT) of the two-photon excited state is determined by exploiting the temporal delay in the pulse sequence. The effect of the pulse delay and the fs pulse bandwidth on the system dynamics is discussed as well as the role of dephasing collisions between K and buffer gas atoms. The proposed scheme can be employed in radiative multi-level systems, for the direct estimation of coherence relaxation rates of various states.

Enhanced violation of the Collins-Gisin-Linden-Massar-Popescu inequality with optimized time-bin-entangled ququarts

High-dimensional quantum entanglement is drawing attention because it enables us to perform quantum information tasks that are robust against noises. To test the nonlocality of entangled qudits, the Collins-Gisin-Linden-Massar-Popescu (CGLMP) inequality has been proposed and demonstrated using qudits based on orbital angular momentum, time-energy uncertainty, and frequency bins. Here, we report the generation and observation of time-bin entangled ququarts. We implemented a measurement for the CGLMP inequality test using cascaded delay Mach-Zehnder interferometers fabricated by using planar lightwave circuit technology, with which we achieved a precise and stable measurement for time-bin-entangled ququarts. In addition, we generated an optimized entangled state by modulating the pump pulse intensities, with which we can observe the theoretical maximum violation for the CGLMP inequality test. As a result, we successfully observed a Bell-type parameter $S_4 = 2.774 \pm 0.025$ violating the CGLMP inequality for the maximally entangled state and an enhanced Bell-type parameter $S_4 = 2.913 \pm 0.023$ for the optimized entangled state.